ANALYSES OF THE PASSIVE FILMS ON STAINLESS STEELS BY MODULATION SPECTROSCOPY.

In order to gain a better understanding of the change in the chemical composition of surface films on stainless steels as a function of the alloy composition or electrode potential, surface analyses by modulation spectroscopy were performed on a series of FE-Cr alloys (5 to 70% Cr), 20Cr-25Ni stainless steel and pure metals constituting these alloys (Fe, Cr and Ni) at various anodic potentials in 1m-Na//2SO//4 solution (pH 2. 00). It was found that a sharp and substantial increase in Cr**3// plus ions in the film of Fe-Cr alloy occurs at potentials in the passivity region with increasing Cr content of the alloy up to 20%. The film composition continuously changes with electrode potential, e. g. , the film formed on Fe-20Cr alloy at potential in the passivity region is mainly composed of chromic oxides and in the secondary passivity region, of ferric oxides.